Combustion exhaust gas exhausted from a boiler, an internal combustion engine, etc. contain a nitrogen oxide and a sulfur oxide that are sources of air pollution.
Art recognized processes for removing nitrogen oxide from exhaust gases include the methods of (1) oxidizing nitrogen monoxide to nitrogen dioxide and thereafter washing said nitrogen dioxide with an aqueous alkali solution, (2) dissolving such nitrogen oxide in a nitrogen gas by contact reduction of nitrogen monoxide, and (3) causing nitrogen monoxide to be absorbed and adsorbed into a solid.
However, all of these methods (1) - (3) are directed to treatment of an exhaust gas containing a high concentration NO.sub.x ; and it has been difficult to efficiently remove a low concentration NO.sub.x from an exhaust gas containing the same.
An effective method for removing sulfur oxide from exhaust gases is a method of wet-desulfurizing an exhaust smoke; a technology of concurrently removing a mixture of a sulfur oxide and a nitrogen oxide as well utilizing said method is under development; however, the efficiency of that process is still very low.